The present invention relates to a lead frame for an integrated circuit (hereinafter "IC") especially for a molded type IC package. More particularly, the present invention relates to a lead frame for an IC which enables the width of an inner lead wire to be as fine as possible, so that the packing density of the IC can be increased. The invention is applicable to plastic IC packages and the like.
A lead frame for an IC comprises a stage on which an IC die or chip (hereinafter "chip") is mounted, and lead pins are wired to the chip for connecting the chip to external circuits. The stage and lead pins are fixed to each other and held in position by stage bars and tie bars respectively, and also fixed to an outer frame. Lead frames are generally made in long strips by punching out a pattern from a metal ribbon or a roll of sheet metal.
An IC chip is usually mounted on the stage of a lead frame and bonding pads, formed on the chip, are electrically connected with bonding wires to corresponding tips of the lead frame, the other end of the lead frame corresponding to lead pins of the IC. The combination is then molded in a plastic material and separated from the roll of lead frames by cutting off the lead pins from the lead frame (i.e., from the tie bars) and consequently from the outer frame. Shaping the lead pins that project out of the molded case finishes a mold type semiconductor device.
The portion of the lead frame that is encapsulated in the molded case is called an inner lead, and the portion that projects out of the molded case is called an outer lead or lead pin.
Today, the trend is to package larger and more complex chips in standard size packages; thus, increasing the packing density of the ICs. The ratio of the chip area or stage area to the total area occupied by the IC package is defined as the packing density factor. Accordingly, a high packing density factor indicates packages occupying a smaller area than packages having a small packing density factor.
To increase the packing density factor, it is necessary to reduce the area occupied by the inner leads. However, as will be described later, the width of the inner leads has a practical lower limit, thus making it difficult to increase the packing density factor by narrowing the inner leads. Moreover, because the bonding pads of chips tend to be arranged along opposing sides of the chip, and because the outer leads must be positioned along the sides of the IC package and aligned perpendicular to the plane of the chips, the inner leads must be bent and formed into complicated shapes. This decreases the packing density factor. Because of the trend toward larger and more complicated chips, the limits associated with conventional packaging techniques have become a serious problem.